Adapted to a mobile station, there is known in the art a clock system that operates the whole time with high frequency. One drawback of this system is that the clock system uses a considerable amount of energy, whereby it is not suitable especially for small and lightweight mobile stations like mobile phones. In order to save the battery of the mobile station, a new clock system, a so called sleep clock system has been developed.
One sleep clock system known in the art is disclosed in the Finnish patent application no 923976 filed earlier by the same applicant. The sleep clock system comprises two clock devices, the first of which is arranged to act with high frequency and the second with low frequency. The first clock device, the normal clock, operates on the megahertz band and the second clock device, the sleep clock, on the kilohertz band. The first clock device is switched off and the second clock device with lower accuracy is employed instead, when the mobile station is in a rest condition. By this arrangement the power consumption can be significantly decreased.
The sleep clock system described above is used e.g. in mobile stations of the GSM (Global System for Mobile Communications) system based on time division multiple access (TDMA). When the normal clock device of the mobile station's sleep clock system is switched off, the receiver of the mobile station is arranged to receive radio signals from the base station only in certain predefined time stamps. The sleep time of the mobile station between two pages, said page being the time it is prepared to recognize an eventual incoming call, is relatively long, in the range of 1 to 2 seconds. This time slot must be accurately measurable.
One problem of the present mobile stations, in which the sleep clock arrangement is used, is that there is generally used a crystal as a heart of the second clock device for synchronizing it. The crystal is always involved with frequency jitter that is in this application typically in the range of 0.5 .mu.s. In a GSM mobile station this corresponds to about 1/8 of a bit period.
In addition, one problem is that the frequency of the crystal changes according to the temperature changes. The nominal frequency is in general achieved at a temperature of 25.degree. C. It can be stated that the bigger the temperature difference compared e.g. with the nominal temperature of 25.degree. C. is, the bigger is the frequency shift. A mobile station must operate reliably in a temperature range from -10.degree. C. up to +55.degree. C. Based on the maximum change speed of the frequency, and taking into account that the sleep time between two pages in the GSM system is max. 2.12 s, it can be estimated that a timing error between the beginnings of two pages is in the worst case about 6.2 .mu.s/.degree. C. Further, if we estimate that the present lightweight mobile stations can warm/cool e.g. 1.degree. C./min, the resulting timing error for a period of one minute is about 6.2 .mu.s. When that is divided by the paging frequency, the resulting time error in the worst case between two pages is 0.2 .mu.s. Even though the time error between two pages is not big, it is cumulative. In case the frequency shift, that is, the cumulative timing error is not compensated by some means, it will soon make the communication system between the mobile station and the base station fail.
In addition to the above mentioned crystal, another problem is involved with the sleep clock arrangement. A timing error of the same kind as the frequency jitter is generated when the clock frequency of the sleep clock system is changed. The second clock of the sleep clock system, in other words, the sleep clock, operates in the kilohertz band, whereas the normal clock operates in the megahertz band. The first clock operating with higher frequency must be on before the clock frequency can be changed from the second clock to the first one. The change of the clock frequency is an asynchronous process. In connection with each change, the stabilization of the normal clock takes some time. The timing error related to the change is in that case half of a clock cycle of the normal clock, that is, about 0.5 .mu.s. The start of a paging function is also an asynchronous process. The signal receiver of the mobile station cannot be sure that the received paging signal really starts at the edge of the signal of its normal clock. Accordingly, this also involves a timing error that is a half of a clock cycle of a normal clock, that is about 0.5 .mu.s.
Based on what has been stated above, it can be estimated that in the worst case the total timing error between two pages will be formed as follows:
______________________________________ Frequency jitter of the crystal 0.5 .mu.s Temperature roaming of the crystal 0.2 .mu.s Change of clock frequency 0.5 .mu.s Starting point of page 0.5 .mu.s 1.7 .mu.s/page ______________________________________
So, the total timing error between two pages is 1.7 .mu.s, that is about a half of a bit period of the GSM network.
The channel conditions have a significant influence on how well the radio frequency (RF) receiver can receive an RF signal transmitted from the base station. When the mobile station operates during the page with the normal clock frequency, the variations and changes of the channel conditions can be fairly well estimated and so the RF signal transmitted by the base station can be received by the receiver of the mobile station. When the timing error of the own internal clock system of the mobile station must also be estimated in the mobile station, the situation will be changed. The RF signal receiver of the mobile station can't be sure that the page transmitted by the base station starts at the very moment that is indicated by its own timing device. The eventual frequency error of the sleep clock system can also look like an error caused by bad channel conditions.
When the channel conditions are bad, in other words, when there are deep fades and a lot of reflections (caused e.g. by a hilly landscape and a Doppler effect) in the received RF signal, the clock system of the mobile station easily fails in synchronizing. The elements of the mobile station can't judge, if the timing error is caused by bad channel conditions or by the clock system. The elements of the mobile station aim at keeping up the timing signal of the mobile station and its synchronizing in respect to the base station clock. Based on this, a correction signal is tried to be estimated in order to correct the long term timing error, in other words, the frequency error. Bad channel conditions can be expressed as a frequency error of the internal timing of the mobile station. This very soon makes the mobile station to loose the synchronizing to the signal frame structure with respect to the base station. It must be noticed that the sleep clock system is not stable in bad channel conditions, even if an optimized receiver construction like the Maximum Likelihood Sequence Estimator (MLSE) were used.